In this first year of the project, studies of the developing superior laryngeal nerve (SLN) in kittens below two months of age indicated a relatively mature nerve in the neonate as compared to hindlimb nerves with the largest fibers approximately equal to 5.0 mu and the myelinated fibers comprising less than 50% of the fibers. Electrical stimulation of the SLN in kittens as young as five days evoked plurisynaptic brain stem reflexes innervating intrinsic laryngeal muscles (i.e., recurrent nerve) and tongue musculature (i.e., hypoglossal nerve). Although swallowing could not be evoked readily until 30 days postnatually, apnea was consistently demonstrated with low intensities. Implementing these studies of the upper respiratory tract, piglets were subjected over their first two month perod to repeated exposures of three test gases eliciting hyperoxia (100% O2), hyposiz (10% O2), and hypercapnia (6% CO2). The animals demonstrated three stages in their ventilatory responses with maximum responses below 10 days of age, a transitional period between 10-30 days, and a steady state above 30 days. Hypercapnia caused marked hyperventilation as seen in the adult; hyperoxia and hypoxia elicited paradoxical responses. While the majority of the postnatal animals demonstrated paradoxical hypoventilation to hypoxia below 15 days of age with hyperventilatio above this age range, two animals exhibited hypoventilation throughout the two month period. Although the three test gases evoked more sighs in the youngest animals, repeated exposures to hypoxia (for five minute periods) over the two month period did not lead to apnea or termination of respiration.